1. Field of the Invention
The present invention relates to a method for generating a three-dimensional sheet-metal model on a computer screen using a CAD. More specifically, the present invention relates to a method and a computer program for generating a three-dimensional sheet-metal model on a computer screen using a CAD for generating a solid model by a general feature without a sheet-metal attribution.
2. Description of the Prior Art
When generating a three-dimensional sheet-metal model on a computer screen using a CAD, it is necessary to think about an automatic development and an actual machine process. For example, knowledge is required that is different from a usual three-dimensional modeling and is related to shapes including a “bending portion”, an “abutting portion” and a “sheet division” depending on a thickness and a material of the sheet-metal.
A method for generating a three-dimensional model from a two-dimensional CAD drawing is disclosed in Japanese unexamined patent publication No. 11-202919 for example. Conventionally, it is common to generate a three-dimensional sheet-metal model by one of two methods (approaches) as below.
In the first method, a user of the CAD intends to make a sheet-metal model from the beginning and sets sheet-metal attributions before starting the generation of a model for a sheet metal. The CAD process makes the user designate sheet-metal attributions such as a material and a thickness of the sheet metal to be processed for the sheet-metal model, and a special model having parameters thereof is generated to be a three-dimensional sheet-metal model. The parameters that are necessary for operations of “bending portion” and “sheet division” that are performed on the screen by the user are determined in accordance with the sheet-metal attributions that were set first. Therefore, a general solid model without sheet-metal attributions cannot be processed.
In the second method, a general solid model is generated first without considering a sheet metal, and after that the solid model is processed to be a sheet-metal model. For example, a shell model that is one type of a solid model can be generated by an operation of hollowing (boring) a block model with remaining a wall of a designated thickness. After that, shapes necessary for a sheet-metal model such as a “bending portion”, an “abutting portion” and a “sheet division” are generated.
In the above-mentioned two methods for generating a three-dimensional sheet-metal model using a CAD, the first method is closer to human thinking than the second method is. Namely, the method in which the user of the CAD intends to make a sheet-metal model from the beginning and sets sheet-metal attributions before generating a model for the sheet metal is closer to human thinking because it is closer to a real machine process of bending a sheet metal. However, this method has some disadvantages as follows.
Namely, a user of a CAD has to set necessary sheet-metal attributions (such as a thickness and a material of the sheet-metal) considering generation of a sheet-metal model from the start of the modeling. In addition, if the existing model is a model generated by another CAD system or a model without sheet-metal attributions, it is difficult to process the model so as to make a sheet-metal model because of limitation of compatibility between the CAD systems.